This invention relates generally to heat exchanger coils and, more particularly, to a method and apparatus for expanding and belling many different configurations of plate fin heat exchanger coils having either copper or aluminum tubes.
A plate fin heat exchanger coil is commonly constructed with a plurality of flat, parallel plates having laterally spaced holes therein for receiving refrigerant tubes, or hairpin tubes, therein. At each end of the plate fin bundle, there is a tube sheet composed of heavier material, and adjacent the upper tube sheet, the open ends of the hairpin tubes are fluidly connected by way of U-shaped return bends that are secured thereto by way of soldering or the like. When the coils are installed into a refrigeration system, the refrigerant is made to flow through the hairpin tubes, and the air to be cooled or heated is made to flow over the plate fins, such that a heat transfer is thereby affected.
As known, the tube compression expansion process is generally carried out by expanding the entire coil at one time by passing tube expanding rods through the open ends of the hairpin tubes and then belling the tube. A backing plate is placed against the hairpin tube bends during compressive expansion to prevent the tubes from being driven out of the unit as the expanding tools are forced therethrough. As a result of this holding action the tubes are compressed rearwardly as they are being expanded outwardly by the tools. This in turn, causes the tubes to shrink so that the axial length of each tube can vary dramatically in final assembly. Because of the differences in tube length, belling of the tubes is difficult and generally results in uneven or misaligned bells being formed in the tube ends. The return bend therefore cannot be properly seated within the bells leading to the formation of relatively weak or incomplete solder joints in this critical region.
In order to better facilitate the formation of the tube bells and the joining of the return bends therein, it has been the common practice in the art to bring the open ends of the hairpins a considerable distance out from the adjacent tube sheet. The additional length of tube allows each bell to be brought to full depth without interference from the tube sheet and also permits the return bend joint to be completely immersed in a solder bath, if the tube is aluminum, to insure that solder will flow into the entire joint region without wetting the tube sheet. The unsupported length of tube between the bell and the tube sheet, however, represents the weakest section in the unit. Hydrostatic tests have shown that the flow circuit will generally rupture in this region when exposed to high internal stresses. Beyond weakening the unit the added length of tubing wastes costly material and thus raises the cost of each unit. Furthermore, the added tube length makes it difficult to compact the unit which in the case of a room air conditioner is of primary importance.
In order to improve the hydrostatic burst strength of a plate fin heat exchanger, a technique for tension expanding hairpin tubes into a fin pack unit, as explained in greater detail in U.S. Pat. No. 4,584,765 was developed. This prior technique was generally carried out on a coil which was first belled by a split collet and pin arrangement, and then expanded. Because of the closeness of the heat exchanger tube rows the bells that were formed are simple, single diameter bells. Further, because of the closeness of the heat exchanger tube rows, the jaw members of the tue clamping fixture were relatively long, thin members. In operation, however, these long, thin jaw members bowed or bent in the transverse direction to the direction of the movement of the expanding bullet during the expansion process. Furthermore, the single diameter bell was not suitable for ultrasonic soldering, but had to use pretinned return bends. Thus, to use ultrasonic soldering, the bell diameter would have had to be increased, which would have required even less space between adjacent tubes, making the jaw member even thinner and subject to more bowing.
The major disadvantage, however, of the prior belling and expanding devices is that they were not very flexible in manufacturing coils of different configurations. As a consequence, the manufacture of a variety of different coils is time consuming and costly due to the set-up time necessary to change from one coil configuration to another coil configuration.
Thus, there is a clear need for an expander of the compression type or tension type which is very flexible in that through automatic or quick manual changes, many different configurations of plate fin coils having either copper or aluminum tubes can be manufactured.